1. Field of the Invention
This invention relates generally to chopper-stabilized amplifiers, and more particularly, to means for reducing offset and ripple in such amplifiers.
2. Description of the Related Art
Operational amplifiers are ubiquitous in electronic circuitry. In some applications, it is essential that an op amp have a very low input offset voltage. Two techniques are commonly employed to achieve this: auto-zeroing and chopper-stabilizing. However, both of these techniques have drawbacks. For example, auto-zeroing can result in increased in-band noise due to aliasing, whereas chopper-stabilizing can result in ripple at the chopping frequency appearing in the output voltage.
A conventional chopper-stabilized amplifier is shown in FIG. 1. A set of chopping switches 10, 12 chop the input applied to a transconductance amplifier Gm1, a set of chopping switches 14, 16 chop the output of Gm1, and an output amplifier Gm2 integrates the chopped output of Gm1 to produce the amplifier's output Vout. The chopping switches are operated with complementary clock signals “Chop” and “Chop_Inv”; switches 10 and 14 are closed and switches 12 and 16 are open when “Chop” is high, and switches 10 and 14 are open and switches 12 and 16 are closed when “Chop_Inv” is high. Ideally, the input offset voltage of Gm1 is zero, in which case chopping switches 10 and 12 convert the input voltage to an AC signal, and switches 14 and 16 convert the AC signal back to DC, such that no ripple is present in Vout. However, in practice, Gm1 will have a non-zero input offset voltage, represented in FIG. 1 as a voltage Vos1. This results in a ripple voltage being induced in Vout, with frequency components appearing in the output spectrum at the frequency of the chopping clocks and multiples thereof (as shown in FIG. 1).
Several methods have been used to reduce chopping-related ripple associated with a chopper-stabilized amplifier. One method, described in A. Bakker and J. H. Huijsing, “A CMOS Chopper Opamp with Integrated Low-Pass Filter”, Proc. ESSCIRC, 1997, employs a sample-and-hold (S/H) circuit in the signal path; ripple is reduced by sampling the signal every time the waveform crosses zero. However, the S/H circuit adds an additional pole to the amplifier's frequency response, and makes frequency compensation difficult.
Another approach is discussed in U.S. Pat. No. 7,292,095 to Burt et al., in which a switched capacitor notch filter is inserted into the amplifier's signal path following the chopping switches, with the filter operated so as to reduce ripple. However, ripple present on the input side of the filter can be coupled to the amplifier's output via a compensation capacitor.
Yet another technique uses a feedback loop to suppress ripple in the signal path that arises due to an input offset voltage associated with the transconductance amplifier which receives the chopped input signal; this approach is illustrated, for example, in K. A. A. Makinwa, “T4: Dynamic Offset-Cancellation Techniques in CMOS”, ISSCC 2007, p. 49. However, no means is provided to suppress an input offset voltage associated with the loop's feedback amplifier.